Low NOx coal fired burners employ various types of hardware in the burner nozzles to alter the primary air/pulverized coal (PA/PC) stream before entering the burner throat for initiation of combustion. These devices are designed to enhance fuel/air mixing to better control NOx emissions. U.S. Pat. No. 4,380,202 discloses a conical diffuser 10 (an example of which is depicted in FIG. 1) that has been utilized in Babcock and Wilcox's DRB-XCL® and DRB-4Z® burners. The conical diffuser 10 is located near the entrance of the coal nozzle 4 downstream of an optional stationary deflector 8 located within the annuals of coal nozzle 4. The diffuser promotes the generation of a fuel rich ring of fuel near the walls of the coal nozzle 4 downstream of the conical diffuser 10, thereby promoting improvements in flame stability and lower NOx emissions. The conical diffuser 10 is typically constructed from ceramic materials to improve wear resistance.
Combustion testing has demonstrated that an air-staged DRB-4Z® burner equipped with a distribution cone 5 located in the coal nozzle 4 and upstream of a standard bladed impeller 12 (see FIG. 2) produces lower NOx emissions than the same burner equipped with a conical diffuser. Testing with eastern bituminous coal has shown that a NOx reduction of about 17% can be achieved using a standard bladed impeller 12 when staged near 0.8 stoichiometry. Increased near field mixing under reducing conditions tends to favor lower NOx emissions. Field testing has also demonstrated lower NOx emissions are achieved with DRB-XCL® burners equipped with standard bladed impellers 12 compared to those equipped with conical diffusers 10 under certain staged conditions.
While standard bladed impellers 12 and similarly located mixing devices can offer functional NOx improvements, they generally suffer from erosion and high temperature related degradation. Achieving the intended mixing benefit of standard bladed impellers 12 generally requires placing the impellers at or near the exit of the burner coal within the coal nozzle 4. However, at these locations impellers readily reach high temperatures from radiative heat transfer from the furnace. These high temperatures are undesirable to impeller longevity as they can thermally erode metal components directly and/or cause coal to stick and cake upon the device causing additional unfavorable consequential damages.
Pulverized coal is highly abrasive, and erosion from pulverized coal is a consistent problem for burner component in direct contact therewith. While ceramics can minimize this effect and are frequently used to protect equipment from erosion, high temperatures near the exit of the burner coal nozzle 4 prevent the effective use of ceramics in such applications. When combined, erosion and exposure to high temperatures generally shorten component life of impellers and similarly located devices to typically about a year of effective service life, after which the burner experiences diminished performance until such time that the impeller is replaced. Standard impellers 12 and similarly located devices thus experience a limited effective service life in the power generation, requiring substantial expenditures (cost, material, labor, and outage time) to facilitate repeated replacements. A need thus exists to develop a diffuser impeller device of a lengthened service life to alleviate concerns associated with prior art impellers.
An additional concern of pulverized coal fired burners is the potential for non-uniform distribution of pulverized coal and primary air to multiple burners served by a given pulverizer. Such non-uniformities are due in part to differences in coal piping from the source (pulverizer) of a pulverized coal stream to each individual coal outlet (burner). Each burner, as provided within a given boiler/combustion facility, is located at a unique distance from the pulverizer that supplies the pulverized coal to the burner. Inherent in any given boiler facility are differences such as: lengths of coal piping runs, number of bends per each run, bend geometries, and in some cases a single mill or pulverizer can supply multiple elevations of burners. These factors combine to cause differences in flow resistance unique to each pipe, and thus each burner. To compensate for these differences, fixed orifices or similar devices are sometimes utilized in an effort to balance flow distribution through each of the coal pipes for each pulverizer. While helpful, such devices have inherent limitations making it not possible to provide sustainable uniform distribution.
Another technique is to apply adjustable flow resistors in the coal piping. Adjustable flow resistors provide the advantage of on-line adjustment for measured imbalances, with varying effectiveness. However, such devices are generally economically infeasible based on the need to supply a ceramic lined spool piece to house such a device. Further, installation costs provide an additional barrier to feasibility due to the need for coal piping alterations (cutting, addition of flanges), a lack of accessibility, and a need for new platforms etc. to install and maintain such equipment. A need thus exists for improved readily installable adjustable flow resistors.
Effective impeller designs must also take into consideration various characteristics of combustion such as flame length. Low NOx pulverized coal-fired burners tend to form long flames and produce higher levels of unburned combustibles relative to conventional burners. Long flames generally result from insufficient air supply to the fuel jet as it proceeds into the furnace. Secondary air from the outer air zones of low NOx burners does not effectively penetrate the fuel jet, such that uncombusted fuel persists along the flame axis. Many low NOx systems utilize over-fire air ports to burn out uncombusted fuel in a manner that inhibits NOx emissions via the well known principle of air staging.
Depending on a given furnace's dimensions (depth, height, etc. . . . ), excessively long flames can result in flame impingement, slagging, and corrosion of boiler tubes thus impairing the function of the burner. Longer burner flames may also unfavorably extend into portions of the furnace where over-fire air is introduced through overfire air ports. In such instances the ability to control NOx formation is unfavorably inhibited as air supplied by the overfire air system can extend the flame beyond the over-fire air zone, thereby effectively merging multiple combustion stages and minimizing the benefits of stage combustion. Effective mixing of coal and air prior to combustion provides a degree of control over flame length. An industry need thus exists to provide a diffuser impeller of improved wear resistance; thereby enhancing controlled air/fuel mixing, and thus resulting flame and combustion characteristics, associated with an operative diffuser impeller.